Latest Updates from 2025 CCTF Meeting
The Consultative Committee for Time and Frequency (CCTF) met recently to review progress on key issues in time and frequency metrology. The meeting brought together experts from around the world to assess advances in optical frequency standards, the future of Coordinated Universal Time (UTC), and new timekeeping requirements for space exploration.
Watch the highlights, as seen by the CCTF President, Dr Noël Dimarcq, recorded following the meeting.
Highlights from the CCTF meeting at the BIPM
1. Towards a new definition of the second
Work is progressing on the evaluation of optical frequency standards for a future redefinition of the second. Several international and regional comparison campaigns are underway and planned, helping to assess candidate transitions and consolidate uncertainty budgets across laboratories. Depending on results and consensus, a decision could be considered around the end of the decade.
2. Continuous UTC – without leap seconds
In the near future, Coordinated Universal Time (UTC) will become a (quasi) continuous time scale, eliminating the discontinuities introduced by the leap second.
In the months preceding this meeting, the Consultative Committee for Time and Frequency (CCTF) and the International Earth Rotation and Reference Systems Service (IERS) examined the possible need for the first negative leap second, due to the recent higher rotational speed of the Earth. User communities also independently assessed the potential disruptive impacts that such a change could have.
During the meeting, the CCTF shared this information and, in response to concerns formally raised by users, agreed to work towards a fast implementation of continuous UTC by encouraging international agreement — with the goal of avoiding the risks associated with a negative leap second.
Context: avoiding the risks associated with a negative leap second
Concerns raised around the introduction of a negative leap second include the risk that some synchronization systems might not apply the adjustment correctly — or at all — or that they could crash and potentially lead to one-second timing errors. Even such a small discrepancy could disrupt global navigation, telecommunications, internet services and other critical infrastructure that depend on precise timekeeping. The international time community is therefore working together to avoid these risks through further coordination and discussion.
3. Timekeeping for lunar navigation
The CCTF is contributing to ongoing studies with space agencies on a reference time scale for future positioning, navigation and timing (PNT) services around the Moon. A series of workshops with lunar system providers is planned over the coming months to evaluate options and prepare recommendations for consideration by the General Conference on Weights and Measures (CGPM).
The 2025 CCTF meeting reaffirmed the vital role of international collaboration in advancing time and frequency metrology. From redefining the second to preparing for space-based timekeeping, the CCTF’s work helps ensure that global measurement standards remain robust and ready for the challenges ahead.
About the CCTF
The CCTF advises the CIPM on matters related to time and frequency, including the realization and dissemination of UTC and the second, coordination of international comparisons, and the development of future standards. Its work supports the internationally recognized Calibration and Measurement Capabilities (CMCs) published in the BIPM Key Comparison Database (KCDB), underpinning reliable timekeeping for science, technology and global infrastructure.
Further viewing: expert perspectives from BIPM150
Explore concise interviews and talks on optical clocks, redefining the second, and future challenges for global metrology.
Nobel Laureate William D. Phillips
Optical Clocks and the Next SI Second
Dr Pascale Defraigne
Why We Need to Redefine the Second and Time on the Moon
Future Challenges for Global Metrology
Perspectives from the BIPM150 Symposium